Field of the Invention
The present invention relates to optical fibers, and more particularly to the finishing of the end face of an optical fiber.
Description of Related Art
There are many advantages to transmitting light energy via optical fiber waveguides and the use thereof is diverse. Single or multiple fiber waveguides may be used simply for transmitting visible light to a remote location. Complex communication systems may transmit multiple specific optical signals. These devices often require the coupling of optical fibers in end-to-end relationship, to effectively place the end faces of the two optical fibers in face-to-face juxtaposition with little or no space between them. This face-to-face optical coupling represents a source of light loss. The cleaved end of the optical fiber and the waveguide at the center of the fiber should be smooth and defect-free.
If the ends of the fiber are uneven, excessive light loss can result due to reflection and refraction of light at the cleaved end surface (e.g., a splice or juncture region). Roughness of the optical surface presents limits in the efficiency of optical signal transmissions by both scattering light and introducing contact gaps in the mating of connectors. When placing optical fibers in end-to-end relationship, to minimize light loss from back reflection from the interior of the end face of the transmitting fiber and to minimize insertion loss, e.g. the loss of signal strength in transmission from one fiber to the other, the cleaved fiber end face needs to be smooth and scratch free as possible to optimize optical coupling between fibers (e.g., in demountable connectors, permanent splices and photonic devices).
Given the imperfections created at the cleaved ends of the fibers, current termination approaches involve conventional cleaving of the optical fiber followed by mechanical polishing of the resultant end face to eliminate imperfections of the cleaved face non-planar form. Traditional methods using various grinding and polishing steps have been around for decades as a means to remove surface roughness. Finishing of the end faces of the fibers involves a laborious procedure which entails placing a quantity of an uncured epoxy in the passage of a zirconia terminal ferrule into which the fiber is to be inserted. The fiber is inserted into the ferrule and pushed through the epoxy at the end face of the ferrule to ensure that at least some of the epoxy is pushed out beyond the end face of the ferrule to provide a firm support for the optical fiber while it is hand-polished to smooth its face. After positioning the fiber and before polishing, the epoxy is cured for about twenty-four hours. An end portion of the fiber that protrudes from the ferrule and cured epoxy is broken off close to the end face of the ferrule. A small portion of the fiber, after being broken off, protrudes from the end face of the ferrule and is firmly supported by the cured epoxy. The epoxy also projects a small distance from the end face of the ferrule, and also supports the fiber within the ferrule passage. This rigid support for the fiber is necessary to enable it to withstand the abrasive action of an abrasive disc that has one of a predetermined number of different hardness. This disc is pressed against the end of the fiber to indent the disc face a small amount by the pressure of the fiber end. The disc is caused to slide over the end face of the fiber. This abrasive disc grinding is accomplished by hand or with automated machinery, and it is interrupted on occasion to inspect the end face and to measure the amount of back reflection. Disc faces and disc hardness may be changed during this procedure.
U.S. Pat. No. 4,979,334 disclosed an automated mechanical polishing device that houses numerous connectors. However, mechanical polishing processes cause the surface of silica glasses to form a thin layer of glass with a higher index of refraction that is only hundreds of nanometers thick; this phenomena was originally reported by Lord Rayleigh in 1937 [L. Rayleight, “The surface layer of polished silica and glass with further studies on optical contact”, Proceedings A of the Royal Society, V. 160, No 3, 1937]. This thin layer of glass with a larger index of refraction causes light to reflect back from the interface between two polished optical fibers mated in fiber-optic connector. Attempts have been made previously to improve the surface finish and/or remove the damaged layer with high index of refraction on an optical fiber end surface by methods other than the standard mechanical polishing. U.S. Pat. No. 5,226,101 disclosed a schematic for laser polishing a fiber using a CW (continuous wave) CO2 laser. Udrea et al. reported achieving an improvement of surface roughness from 2.5 μm to less than 100 nm with a loss decrease of 1.5 dB after laser irradiation [M. Udrea, H. Orun, “Laser polishing of optical fiber end surface,” Optical Engineering 40(9), 2026-2030 (2001).]. Alternate designs that implement laser cleaving with additional polishing steps have also been designed. US2005/0008307A1 discloses a technique to thermally shape the end face of optical fiber that has had the cladding removed. U.S. Pat. No. 7,082,250 disclosed a technique to laser cleave a connectorized fiber (i.e., an end of the optical fiber is fixed in a ferrule, and the optical fiber is not demounted from such ferrule after final polishing of its end face; the optical fiber and ferrule may be assembled into a connector) and then perform an additional step of fine mechanical polishing.
While the prior art laser shaping/polishing processes may reduce high back reflections by reducing surface roughness, a high index layer is still being introduced, which are often viewed as fundamental problems. The melting of the fiber material at the fiber end face creates a convex surface having a radius of curvature on the order of the radius or diameter of the fiber. This results in undesirable optical effects, unless this convex surface is further shaped by mechanical grinding. Further, polishing not just bare fiber but connectorized fiber still remains an issue. A major concern in laser polishing of a connectorized fiber is the epoxy used and the material properties of ceramic. Epoxy typically absorbs the laser energy and subsequently melts, burns, or vaporizes at the high power densities required to soften or melt the fiber; this which contaminates the end face of the optical fiber and needs to be removed and it compromises the retention of the fiber within the ferrule. Ceramic ferrules also absorb energy from the laser beam, and due to their low thermal conductivity, rapid temperature changes during the laser treatment can introduce cracking in the zirconia (ZrO2) and alumina (Al2O3) ferrules. The energy can even be sufficiently high as to vaporize the end faces of the ferrules, altering their endface geometry. Further, the prior methods require precision alignment of the applied laser beam, and additional fine polishing steps after the application of the laser, which increases burden and complexity in manufacturing, and which could result in a high index layer.
The drawbacks of the prior art fiber end face polishing processes posed significant challenges to meet the stringent specifications set forth by prevailing industry standards, such as GR-326-CORE (Generic Requirements for Singlemode Optical Connectors and Jumper Assemblies, Issue 4). Such standards set forth various requirements, including the polish radius (defined as the radius of the ferrule end-face surface as measured from the ferrule axis), the fiber protrusion and undercut (defined as the distance between the top of the glass fiber as measured against the surrounding material in a spherical plane), the ferrule apex offset (measured as the distance between the spherical center of the polished end-face and the center of the fiber), and angled polish (defined as the angle at which the ferrule end face is polished, relative to the axis perpendicular to the ferrule axis).
Given the shortcomings of the prior art laser polishing processes, it remains desirable to develop an improved laser polishing process to provide for finishing an optical fiber end face while minimizing or avoiding the above-mentioned problems.